The purpose of a pathlength controller in a ring laser gyro is to maintain a constant pathlength. In order to do so, the laser intensity within the gyro is measured with one or two separate photodetectors, each having a corresponding preamplifier connected to its output. By demodulation of the signals from the separate photodetectors, synchronously with jitter imposed on a pathlength controlling piezoelectrically activated movable mirror, the laser can be tuned to the gain center of the laser intensity transition.
In the past, it has been necessary to mount and align the separate photodetectors when installed on a gyro. Further, special connecting leads, capable of flexing, have been necessarily employed to connect the photodetectors to a ring laser gyro undergoing vibration. Customarily, the photodetectors of the pathlength controller are equipped with filters or are otherwise specially manufactured to be insensitive to blue plasma light emitted by lasering gases in the gyro.